Hydrocarbon
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HYDROCARBON
Organic compounds composed of only carbon and hydrogen are called hydrocarbon.
CLASSIFICATION OF HYDROCARBONS
On the basis of structure, hydrocarbons are broadly divided into main types
1. Acyclic or open chain hydrocarbon
2. Cyclic or closed chain hydrocarbon
Acylic hydrocarbon
These hydrocarbon contains open chain of carbon atom in their molecule. These are
also called as aliphatic hydrocarbon
These are further classified as
i)
ii)
iii)
Alkanes: C – C bond ( saturated compounds)
Alkene : C = C bond ( Unsaturated compounds)
Alkyne: C≡ C bond ( Unsaturated compounds)
Cyclic hydrocarbon
These contains closed chain or rings of carbon atom in their molecule
These are further divided in two groups
(i)
(ii)
Alicyclic hydrocarbon: The hydrocarbon which contains ring of carbon
atom in a molecule and resemble aliphatic hydrocarbon in their properties
Aromatic hydrocarbon: These hydrocarbon contains at least one special
type of hexagonal ring of carbon atom with three double bonds in
alternate position.
The major source of aliphatic hydrocarbon are petroleum and natural gas, where as
aromatic hydrocarbons are main obtained from coal
PETROLEUM
(Petra = rock , oleum = oil)
 It is dark viscous fluid with unpleasant smell found at various depth under the
earth crust.
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 Crude petroleum is a mixture of hydrocarbons containing 80-85% of carbon
10-14% hydrogen and small amount of oxygen, nitrogen and sulphur.
Petroleum products
1. Gaseous ( C1 – C4)
Boiling range : 113 – 303 K
Uses: As a full, for production of carbon black
2. Petroleum ether (C5 – C7)
Boiling range : 303 – 363K
Use : As of solvent for rubber, oil, fasts and varnishes, for dry cleaning
3. Gasoline or Petrol ( C8 – C11)
Boiling range : 343 -373K
Uses: As a motor fuel, for dry cleaning
4. Kerosene oil or Paraffin oil ( C12 – C15)
Boiling range : 448 – 548K
Uses: AS an illuminate fuel, just engine fuel and for making oil gas
5. Gas oil, fuel and diesel gas ( C15 – C18)
Boiling range : 573 – 673
Use: Furnace fuel, fuel for diesel engine
6. Lubrication or Mineral oil ( C16 – C20)
Boiling range : 623 K and above
Uses: As a lubricant
7. Grease, Vaseline, petroleum jelly ( C8 – C22)
Use: pharmaceutical preparations
8. Paraffin wax ( C20 – C30)
Uses: In the manufacture of candles, waxed papers and for water proofing.
OCTANE NUMBER
 Quality of petrol or gasoline is graded terms of octane number
 Octane number is defined as the percentage by volume of iso-octane present
in the mixture of iso-octane and n-heptane which has same knocking
properties.
 Metallic sound which arises due to violent jerk in piston is known as knocking
CH3 – CH2 – CH2 – CH2 – CH2 – CH2 – CH3
( n – heptanes , octane number = 0)
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 Higher the octane number of fuel, better is the fuel. Octane number of
gasoline is 80
 Straight chain hydrocarbon have low octane number.
 Presence of multiple bond increases octane number.
 Aromatic hydrocarbon have higher octane number.
 Octane number decreases with increase in chain length but increases with
branching of chain
 Octane number follows the following order
Aromatic hydrocarbons > cycloalkanes > branched chain alkane
> straight chain alkanes
ALKANES
 Alkanes are saturated open chain hydrocarbon containing carbon-carbon
single bond
 General formula of alkanes is CnH2n+2
 Methane has a tetrahedral structure which is multiplanar, in which carbon lies
at the centre and four hydrogen atoms lie at the four corners of a regular
tetrahedron. All H-C-H bond angles are of 109.5O
 In alkanes, tetrahedral are joined together in which C – C and C – H bond
lengths are 154 pm and 112pm
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ISOMERISM OF ALKANES
 Difference in properties due to difference in their structures are known as
structural isomers
 Structural isomers which differ in chain of carbon atoms are known as chain
isomers
Eg. C5H12
I)
CH3 – CH2 – CH2 – CH2 – CH3
Pentane ( n-pentane)
II)
III)
PREPARATION OF ALKANES
1. Hydrogenation of unsaturated hydrocarbons
𝑃𝑏,𝑃𝑑,𝑁𝑖
𝐶𝐻2 = 𝐶𝐻2 + 𝐻2 →
Ethane
𝑃𝑡,𝑃𝑏 𝑜𝑟 𝑁𝑖
𝐶𝐻3 − 𝐶𝐻 = 𝐶𝐻2 + 𝐻2 →
Propene
𝐶𝐻3 − 𝐶𝐻3
ethane
𝐶𝐻3 − 𝐶𝐻2 − 𝐶𝐻3
Propane
 Metals like platinum, palladium adsorbs dihydrogen gas on their surface and
activate the hydrogen bond
 Platinum and Palladium catalyse the reaction at room temperature
 Nickel requires high temperature and pressure
2. Reduction of alkyl halide
𝑅 − 𝑋 + 𝐻2 → 𝑅 − 𝐻 + 𝐻𝑋
Order of reactivity of different alkyl halides is
R – I > R – Br > R – Cl > R – I
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 The purpose of using of red phosphorus is to remove iodine
 1O and 2O alkyl halides are readily reduced by LiAlH4 but 3O halides give alkene
𝐿𝑖𝐴𝑙𝐻4
𝐶𝐻3 − 𝐶𝐻2 − 𝐶𝑙 →
𝐶𝐻3 − 𝐶𝐻3
BUT
 NaBH4 reduces 2O and 3O halides but 1O
3. Wurts reaction
 If two different alkyl halides are used mixture of three different alkanes are
obtained.
Eg
The separation of the mixture into individual members is not easy
 Wurtz reaction is used for the preparation of higher alkanes containing even
number of carbon atoms
Mechanism
Wurtz reaction is supposed to follow free radical mechanism
 Ionic mechanism is suggested for Wurtz reaction
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Reactivity order of alkyl halides towards Wurtz reaction is
1O > 2O > 3O and RI > RBr > RCl
4. Decarboxylation of carboxylic acid.
𝐶𝑎𝑂
𝑅 − 𝐶𝑂𝑂𝑁𝑎 + 𝑁𝑎𝑂𝐻 → 𝑅 − 𝐻 + 𝑁𝑎2 𝐶𝑂3
𝐶𝑎𝑂
𝐶𝐻3 𝐶𝑂𝑂𝑁𝑎 + 𝑁𝑎𝑂𝐻 → 𝐶𝐻4 + 𝑁𝑎2 𝐶𝑂3
Alkane obtained in this method have one carbon less than the parent
compounds
5. Kolbe’s electrolytic method
𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠
𝑅𝐶𝑂𝑂𝐾 + 𝑅𝐶𝐶𝑂𝐾 →
Mechanism
𝑅 − 𝑅 + 2𝐶𝑂2 + 𝐻2 + 2𝐾𝑂𝐻
If number of carbon atoms in potassium carboxylate = n
Number of carbon atoms in alkane formed = 2(n-1)
Therefore, methane cannot be prepared by this method
6. Hydrolysis of aluminium and beryllium carbide
𝐴𝑙4 𝐶3 + 12𝐻2 𝑂 → 4𝐴𝑙 (𝑂𝐻 )3 + 3𝐶𝐻4
Aluminium carbide
𝐵𝑒2 𝐶 + 4𝐻2 𝑂 → 2𝐵𝑒(𝑂𝐻 )2 + 𝐶𝐻4
Beryllium carbide
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PHYSICAL PROPERTIES OF ALKANES
(1) Physical state
Due to weak intermolecular forces, alkanes from C1 to C4 are colourless gas, next
thirteen C5 to C17 are colourless, odourless liquid. Higher members form C18
onwards are waxy solids
(2) Solubility
Alkanes are non-polar, hence insoluble in polar solvents like water but readily
soluble in non-polar solvents like benzene, ether etc.
The solubility decreases with increase in molecular mass
(3) Density
The relative densities of alkanes increases with increase in molecular mass,
but very slowly till it becomes constant at about 0.8. thus all alkanes are
lighter than water
(4) Boiling point
Boiling point of alkane increases with increasing molecular mass
The greater the branching of the chain, lower is the boiling point. This is
because surface area of branched chain alkanes is less than that of straight
chain alkanes. As a result they experience lower magnitude of van der Waals
forces
(5) Melting point
Melting point of alkanes also increases with the increase in molecular weight
Alkanes with odd number of carbon atoms have lower melting point than the
next lower or next higher alkanes having even number of carbon atoms.
(6) Viscosity
Viscosity of alkanaes ( in liquid states) increases with chain length
For isomeric alkanes, viscocity increases with branching
CHEMICAL PROPERTIES OF ALKANES
1) Halogenation
ℎ𝜐
𝐶𝐻4 + 𝐶𝑙2 → 𝐶𝐻3 𝐶𝑙 + 𝐻𝐶𝑙
ℎ𝜐
𝐶𝐻3 𝐶𝑙 + 𝐶𝑙2 → 𝐶𝐻2 𝐶𝑙2 + 𝐻𝐶𝑙
ℎ𝜐
𝐶𝐻2 𝐶𝑙2 + 𝐶𝑙2 → 𝐶𝐻𝐶𝑙3 + 𝐻𝐶𝑙
ℎ𝜐
𝐶𝐻𝐶𝑙3 + 𝐶𝑙2 → 𝐶𝐶𝑙4 + 𝐻𝐶𝑙
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Reactivity of halogens is in order
F2 > Cl2 > Br2 > I2
For a given halogen, abstraction of 1O, 2O,3O hydrogen follows the order
3 O > 2O > 1 O
Reactivity of different hydrogen towards halogenations is
Bemzyl > Allyl > 3O>2O > 1O
Mechanism
Free radical substitution
(1) Chain initiation step
(2) Chain propagation step
And so on……
(3) Chain terminating step
Iodination is very slow and reversible reaction. It can be carried out in the presence
of oxidizing agents like HNO3 or HIO3
CH4 + I2 ⇌ CH3I + HI
5HI + HIO3 → 3I2 + 3H2O
2) Nitration
ℎ𝑖𝑔ℎ 𝑡𝑒𝑚𝑝
𝑅 − 𝐻 + 𝐻𝑂𝑁𝑂2 →
Nitration follows free radical mechanism
𝑅 − 𝑁𝑂2 + 𝐻2 𝑂
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3) Oxidation
(a) Combustion or complete oxidation
3𝑛 + 1
) 𝑂2 → 𝑛𝐶𝑂2 + (𝑛 + 1)𝐻2 𝑂
𝐶𝑛 𝐻2𝑛+2 + (
2
𝐶𝐻4 + 2𝑂2 → 𝐶𝑂2 + 2𝐻2 𝑂
(b) Incomplete combustion
𝑏𝑢𝑟𝑛
2𝐶𝐻4 + 3𝑂2 →
𝑏𝑢𝑟𝑛
𝐶𝐻4 + 𝑂2 →
2𝐶𝑂 + 4𝐻2 𝑂
𝐶 + 4𝐻2 𝑂
(c) Catalytic oxidation
𝐶𝑢,523𝐾,100𝑎𝑡𝑚
2𝐶𝐻4 + 𝑂2 →
2𝐶𝐻3 𝑂𝐻
𝑀𝑜2 𝑂3
𝐶𝐻4 + 𝑂2 →
(𝐶𝐻3 𝐶𝑂𝑂)2 𝑀𝑛
2𝐶𝐻3 − 𝐶𝐻3 + 3𝑂2 →
𝐻𝐶𝐻𝑂 + 𝐻2 𝑂
Methanal
2𝐶𝐻3 𝐶𝑂𝑂𝐻 + 2𝐻2 𝑂
Ethanoic acid
𝐾𝑀𝑛𝑂4
(𝐶𝐻3 )3 𝐶𝐻 →
2-methyl propane
(𝐶𝐻3 )3 𝐶𝑂𝐻
2-methylpropan-2-ol
4) Isomerisation
The process of conversion of one isomer into another isomer is called
isomerisation.
Isomerisation involves 1, 2 shift of hydride or methyl group
5) Aromatization
The conversion of aliphatic compounds into aromatic compounds is known as
aromatization
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6) Reaction with steam
𝑁𝑖,𝐴𝑙2 𝑂3, 1073𝐾
𝐶𝐻4 + 𝐻2 𝑂 →
𝐶𝑂 + 3𝐻2
7) Pyrolysis
The decomposition of a compound by heat is called pyrolysis.
Mechanism of cracking follows free radical mechanism
Use of alkanes
 Methane is used in the preparation of carbon black which is used for making
printing ink, black paint
 Methane in the form of natural gas used for running the cars, buses etc
 Ethane is used for making hexachloroethane which is artificial camphor.
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ALKENES
Alkenes are unsaturated hydrocarbons having double bond ( C = C)
Their general formula is CnH2n
They are also known as olefins since the lower members form oily product on
treatment with chlorine or bromine.
STRUCTURE OF ALKENES
Carbon-Carbon double bond in alkenes consists of one strong sigma (σ) bond due to
head on overlapping of sp2 hybridised orbitals and one weak pi (π) bond obtained by
sideways overlapping of the two 2p orbitals of the two carbon atoms
The double bond is shorter in bond length (134pm) than the C-c single bond (154pm)
The bond enthalpy of double bond is 681kJ/mol which is greater than single bond
(348kJ/mol)
ISOMERISM IN ALKENES
(a) Position isomerism: This is due to the position of double bond in the same
molecular formula
CH3 – CH2 – CH = CH2 ) But – 1- ene)
CH3 – CH = CH – CH3 ( But – 2 – ene)
(b) Chain isomerism: This is due to the difference in the nature of carbon skeleton
CH3 – CH2 – CH = CH2 ( But – 1 – ene)
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(c) Geometrical isomerism : Isomers, which possess the same molecular and
structural but differ in the arrangement of atom or groups in space due to
restricted rotation around the double bonded atom are known as geometrical
isomer and the phenomenon is known as geometrical isomerism
(1) If two identical atoms or groups lie on the same side of the double bond is
called cis-isomer
(2) If two identical atoms or groups lie on the opposite side of the double
bond is called trans-isomer.
Physical properties of cis-trans isomers
(i)
(ii)
(iii)
(iv)
(v)
Melting point of trans-isomer is more than that of cis-isomer
Boiling point : p.pt. of cis-isomer is more than that of trans isomer
Solubility: The solubility viscosity and refractive index of cis-isomer is more
than the trans isomer in a given solvent
Dipole moment: Cis-isomer has higher dipole moment than trans –isomer
Stability: Trans-isomer is more stable but less reactive than cis-isomer
PREPARATION METHODS OF ALKENES
1. By dehydration of alcohol
Ease of dehydration follows the order
Tertiary (3O) > Secondary(2O) > Primary(1O)
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In the dehydration of secondary and tertiary alcohol, when there is a
possibility of formation of two isomers, the hydrogen atom is preferentially
eliminated from the number of hydrogen atom
This is Saytzeff’s rule
Mechanism
Step I
Step II
Step III
Retropinacol rearrangement ( Wagner – rearrangement)
The above reaction can be explained as follows
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2. Dehydrohalogenation of alkyl halides
It involves removal of halogen atom along with β-hydrogen atom on heating
with alcoholic solution of potassium hydroxide
Reactivity order of different alkyl halides
3o > 2o > 1o and RI > RBr > RCl
It follows Saytzeff’s rule
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3. Dehalogenation of vicinal dihalides
NaI in acetone can also be used in place of Zinc
𝑎𝑐𝑒𝑡𝑜𝑛𝑒
𝐵𝑟 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐵𝑟 + 𝑁𝑎𝐼 →
4. Partial reduction of alkynes
(i)
Birch reduction
(ii)
𝐶𝐻2 = 𝐶𝐻2 + 𝑁𝑎𝐵𝑟 + 𝐼𝐵𝑟
Lindlar’s catalyst
5. Kolbe’s electrolytic method
6. Pyrolysis or cracking
7. Dehydrogenation of alkanes
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PHYSICAL PROPERTIES OF ALKENES
1. Physical state
C2 – C4 are colourless gases, next fourteen members (C5 – C18) are liquids and
next higher members are solid at room temperature
2. Meting point
Melting point increases with increase in molecular mass.
3. Boiling point
Boiling point also increases with increase in molecular mass
Like alkanes, branching in alkenes lower the boiling point
4. Solubility
Alkenes are insoluble in water but soluble in non-polar solvents like CCl4,
benzene, ether, etc.
5. Density
Alkenes are lighter than water
6. Dipole moment
Alkenes are generally non-polar but certain unsymmetrical alkenes are slightly
polar.
CHEMICAL PROPERTIES OF ALKENES
1. Addition of dihydrogen (H2) [ Hydrogenation]
𝑁𝑖
𝐶𝐻2 = 𝐶𝐻2 + 𝐻2 → 𝐶𝐻3 − 𝐶𝐻3
The order of reactivity of various alkenes towards catalytic hydrogenation is
CH2 = CH2 > RCH = CH2 > RCH = CHR > R2C = CHR > R2C = CR2
2. Addition of halogens
The order of reactivity of halogens is
Fluorine > Chlorine > Bromine > Iodine
Mechanism
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3. Addition of halogen acids
Mechanism
Step I
𝐻𝑋 ⇌ 𝐻 + +∶ 𝑋 −
Step II
Electrophile (H+) attacks the double bond to form carbocation
Step III
Nucleophile attacks the carbocation to from the alkyl halide
In case of unsymmetrical alkene having group on either side of double bond,
two theoretically possible product is obtained
CH2 = CH2 + HBr → CH3 – CH2Br
In case of unsymmetrical alkenes having groups on either side of double bond,
two theoretically possible products are obtained.
This can be explained by Markownikoff’s rule
Markownikoff’s rule
The rule states that negative part of the addendum ( adding molecule) gets
attached to that carbon atom which possesses lesser number of hydrogen
atoms
Mechanism
Step I
𝐻𝐵𝑟 → 𝐻 + + ∶ 𝐵𝑟 −
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Step II
Since tertiary carbocation is more stable than secondary carbocation,
therefore, the former predominates because it is formed at faster rate.
Peroxide effect or Kharasch effect
Addition of Hbr to an unsymmetrical alkene in the presence of peroxides gives
the product contrary to Markownikoff’s rule. It is also known as antimarkonikoff addition.
Free radical mechanism
Step I : Peroxide ( like benzoyl peroxide undergoes hemolytic fission to give
free radicals
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Step II : Propagation
Step III : Termination
Peroxide effect is observed only in case of HBr and not in HCl and HI
This may be due to the fact that the H-Cl bond being stronger than H-Br bond,
is not cleaved by the free radical.
Whereas the H-I bond is weaker and iodine free radicals combine to form
iodine molecules instead of adding to the double bond
4. Addition of sulphuric acid
Since alkenes are soluble in cold concentrated acid, this method can be used
for separating them from insoluble compounds like akanes and alylhalides.
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5. Addition of hypohalous acid
Hypohalous acid ( HOX) [ Chlorine or bromine + H2O] readily added on alkenes
to form corresponding halohydrins
Mechanism
The order of reactivity HOX is
HOCl > HOBr > HOI
6. Addition of water
This reaction involves formation of carbocation intermediate that may result
into some unexpected product due to carbocation rearrangement
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Mechanism
Step I : Electrophilic attack by hydronium ion (H3O+) on alkene gives an
intermediate carbocation.
Step II : Nucleophilic attack by water on carbocation to yield protonated
alcohol
Step III: Deprotonation to form alcohol
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7. Oxymercuration – demercuration
8. Addition of HNO3
9. Addition of acetyl chloride
10. Addition of nitrosyl chloride ( Tildens Reagent)
NOCl → Cl- - N+ = O
11. Oxidation
(i)
Complete combustion
CH2 = CH2 + 3O2 → 2CO2 + 2H2O + 1411 kJ/mol
(ii)
Oxidation with potassium permanganate
(a) Oxidation with cold dilute neutral or alkaline KMnO4 cold alkaline
KMnO4 is known as Baeyer’s reagent
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This reaction is used for testing presence of unsaturation
As soon as KMnO4 reacts with alkene its pink colour gets discharged
and brown precipitate of MnO2 is obtained
(b) Oxidation with hot KMnO4
12. Ozonolysis
13. Polymerisation
Two or more molecules of an alkene link together to form a new bigger
molecule of higher molecular mass. This process is called polymerization
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Polymers are used for manufacture of plastic bags, toys, pipe, etc
Uses of alkenes
Ethene is used:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
For the manufacture of polythene – a plastic material
For artificial ripening of fruits
As a general anaesthetic.
As a starting material for a large number of compounds such as glycol,
ethyl alcohol etc
For making poisonous mustard gas ( war gas)
For making ethylene – oxygen flame for cutting and welding metals
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